1. Field of the Invention
The present invention relates generally to a circuit for starting a single-phase AC induction motor, and more particularly, to a circuit for starting a single-phase AC induction motor comprising a circuit capable of cutting off a starting current in a contactless system after starting the motor.
2. Description of the Prior Art
In a compressor and various pumps in a refrigerator, a single-phase AC induction motor has been most commonly used. As a circuit for starting this type of single-phase AC induction motor, a starting circuit constructed using an electromagnetic relay has been widely used. However, a contact may, in some cases, be burned, for example, due to a large current flowing at the time of starting the motor. Accordingly, a starting circuit constructed using a positive temperature coefficient thermistor element (hereinafter referred to as a PTC element) in place of the electromagnetic relay has been utilized.
One example of the conventional motor starting circuit using a PTC element is shown in FIG. 2. In FIG. 2, a single-phase AC induction motor 1 has a main winding 2 and an auxiliary winding 3. Respective one ends of the main winding 2 and the auxiliary winding 3 are connected to a power supply 5 through a switch 4, and the other end of the main winding 2 is also connected to the power supply 5. In addition, the other end of the auxiliary winding 3 is connected to the power supply 5 through a PTC element 6.
In the motor starting circuit shown in FIG. 2, a current flows through the auxiliary winding 3 to start the motor 1 by turning the switch 4 on. Immediately after the motor 1 is started, a current also flows through the PTC element 6. Accordingly, the PTC element 6 is self-heated with the elapse of time. This self-heating causes the resistance value of the PTC element 6 to be rapidly increased, so that the current flowing through the PTC element 6 is rapidly reduced. Consequently, the starting current flowing through the auxiliary winding 3 is reduced, thereby to make it possible to lower the power consumption after starting the motor 1.
When the PTC element 6 is self-heated to enter a state where the resistance value thereof is high, however, the current flowing through the PTC element 6 is not zero, although reduced, as shown in FIG. 3. Specifically, a residual current with some degree of magnitude still continues to flow through the auxiliary winding 3 and the PTC element 6. Consequently, several watts of power still continues to be consumed.
Furthermore, after the switch 4 is turned off, the temperature of the PTC element 6 is not rapidly decreased. In order to restart the motor 1 after the operation of the motor 1 is terminated, therefore, the motor starting circuit must wait until the PTC element 6 is cooled. Specifically, return time required until the motor 1 can be restarted is relatively long.
Additionally, the self-heating of the PTC element 6 exerts a thermally adverse effect on components around the PTC element 6, and the life of the PTC element 6 is reduced because the PTC element 6 continues to be self-heated.